For example, JP-A-2003-295614 describes a related art developing cartridge that has a toner hopper for filling toner, and a supply roller and a developing roller disposed sideways of the toner hopper. The supply roller and the developing roller are mutually welded, a nip being formed between them. The toner of the toner hopper is supplied to the supply roller, then supplied to the developing roller at the nip and carried on the surface of the developing roller. The toner carried on the surface of the developing roller is supplied to an electrostatic latent image formed on a photosensitive drum to visualize the electrostatic latent image.
In the related art developing cartridge, the developing roller has a developing roller drive gear, and the supply roller has a supply roller drive gear. The developing cartridge has an input gear which is coupled by a coupling member a motor of the electro-photographic image forming apparatus, and thus a motive power is transmitted from the external motor to input gear through the coupling member. Both the developing roller drive gear and the supply roller drive gear are meshed with the input gear. A driving force from the motor is input to the input gear from the coupling member, and transmitted from the input gear to both the developing roller drive gear and the supply roller drive gear at the same time. Thereby, the developing roller and the supply roller are rotated and driven simultaneously.
In the related art developing cartridge, when a new developing cartridge is used, there is no toner at the nip between the supply roller and the developing roller. Accordingly, since there is no toner to reduce a frictional force at the nip between the supply roller and the developing roller, a higher load is applied to the supply roller that is about to start rotating than during normal rotation (i.e., a state in which the toner exists at the nip).
Since the toner of the toner hopper drifts from the supply roller to the developing roller, it could be advantageous if the supply roller could start rotating before the developing roller in consideration of a backlash between the input gear and the supply roller drive gear.
However, in the related art developer cartridge, it is difficult for the supply roller to start rotating before the developing roller, because in the related art configuration both the developing roller drive gear and the supply roller drive gear are meshed with the input gear. A driving force inputted from the coupling member into the input gear is subdivided into forces applied to the developing roller drive gear and the supply roller drive gear, and the subdivided smaller driving force is transmitted to the supply roller drive gear. Therefore, there is an insufficient driving force transmitted to the supply roller to which the higher load is applied at a time at which the supply roller starts to rotate. This makes it more difficult for the supply roller to start rotating before the developing roller.